The present invention relates to semiconductor devices, and more particularly, to a design of semiconductor devices, such as metal oxide semiconductor field effect transistors (MOSFETs), which have reduced leakage currents, and to a method for fabricating the same.
FIG. 1A depicts a partial top-view of a conventional MOSFET device 10. As shown in FIG. 1A, MOSFET device has a gate 12 over a substrate 22. A source region 14 and a drain region 16 are located in substrate 22 on opposite sides of gate 12. Source and drain regions 14 and 16 are commonly referred to as having a length L that extends in the y-direction and a width W that extends in the z-direction, shown in FIG. 1A. Consistent with that usage, the dimension of a MOSFET gate, such as gate 12, in the z-direction, will hereafter be referred to as its width, and a reference to the ends of the gate will be understood to refer to opposite ends of the gate in the z-direction.
FIG. 1B is a cross-sectional schematic view of the MOSFET device 10. As shown in FIG. 1B, source 14 and drain 16 are formed in a well region 20 in substrate 22. Gate 12 is separated from substrate 22 and thus source 14 and drain 16 by an oxide layer 15. The thickness of oxide layer 15 and the degree of any overlap of the gate over the source and the drain regions can vary. Device 10 may also have dielectric spacers 50 and 55 on two sides of gate 12, and lightly doped drain (LDD) regions 40 and 42 adjacent source and drain regions 14 and 16, respectively. Spacers 50 and 55 help to farther isolate gate 12 from source 14 and drain 16 to prevent the build-up of device capacitance. Device 10 may be isolated from other devices also formed on substrate 22 by dielectric trenches (not shown) at some or all sides of device 10.
Source and drain regions 14 and 16 in substrate 22 are typically regions doped with dopants of a same conductivity type. Well region 20 is typically doped with dopants of a different conductivity type from that in the source and drain regions. LDD regions 40 and 42 are typically doped with dopants of the same conductivity type as in the source and drain regions, but dopant concentrations in the LDD regions are typically much lighter than in the source and drain regions.
A MOSFET device, such as device 10, behaves like a switche: when it is xe2x80x9conxe2x80x9d, i.e., when a sufficient threshold voltage, Vt, is applied to the gate, a channel 18 is formed in a region immediately under oxide layer 15 and relatively large currents flow through the channel between the source and drain. Ideally, when the MOSFET 10 is xe2x80x9coffxe2x80x9d, there is no current flow. In practice, however, there is typically a small amount of unwanted leakage current when device 10 is off. Assuming that Ion is the current that flows between the source and drain of a MOSFET device in the xe2x80x9conxe2x80x9d state, and Ioff is the small amount of unwanted leakage current that flows or xe2x80x9cleaksxe2x80x9d between any two of the source, drain and gate in an off-state of the device, the on/off ratio (Ion/Ioff) of a transistor is a common figure of merit and benchmark for transistor performance comparisons. Higher Ioff values result in lower on/off ratios, and indicate degraded transistor performance.
There are several causes of off-state leakage currents. Parasitic leakage paths between the gate and channel, commonly referred to as sidewall leakage, can result in excessive forward and reverse gate leakage currents. For example, leakage can occur where the gate overlaps or is closely adjacent the drain and the source (commonly referred to as edge conduction leakage). Devices which exhibit high edge conduction and sidewall leakage are characterized by degraded device performance, such as increased off-state power dissipation.
Leakage currents can also result from other sources within a semiconductor device or as a consequence of various device processing steps. For example, in practice processing steps associated with the formation of shallow trench isolation (STI) may result in electrons being trapped near a substrate-nitride interface, inducing sidewall leakage between an isolating trench and a device.
Leakage currents can also occur due to inverse narrow width effect (INWE). INWE is a parasitic phenomenon which lowers the effective threshold voltage as the length of the gate becomes smaller. Device performance, reliability, layout efficiency and yield are known to be degraded by the inverse narrow width effect. The lower threshold voltage, Vt, means higher off-state leakage currents that increase overall power consumption, result in excess heat generation and can cause problems related to the dissipation of excess heat.
Another design and manufacturing concern relates to gate oxide thinning. Gate oxide thinning occurs over the device lifetime due to stresses on the device such as high applied voltage levels, temperature and imperfections in the oxide layer. The phenomenon of the thinning of the gate oxide film increases the likelihood of dielectric breakdown which can adversely affect the operating characteristics of devices. Moreover, the well-known xe2x80x9chot-carrier effectxe2x80x9d can cause damage to the oxide layer by increasing the chances of oxide breakdown, particularly at oxide edges.
As device geometries continue to shrink and threshold voltages continue to scale down, leakage currents have an even greater impact on device performance. Particularly in low power or high temperature applications, leakage currents can represent a significant source of device degradation and performance impairment. The problem of leakage currents is exacerbated by the existence of numerous possible causes of such currents. To resolve the problem requires complex failure identification and analysis. Such failure analysis projects are costly and highly dependent upon the skills and resourcefulness of the individuals conducting the failure analysis. Therefore, a need exists for a device design and fabrication approach that can compensate for manufacturing defects, device degradation or tolerance failures due to leakage currents.
The present invention provides a design of a MOS transistor with reduced leakage current and without adverse device performance that typically accompanies other prior art approaches.
In one embodiment of the present invention, a MOSFET has a thicker gate oxide at one or more ends of the gate, thereby minimizing leakage currents without adversely impacting overall device Vt as would occur if the gate oxide were made uniformly thicker.
The present invention also includes a method for fabricating a MOS transistor where the gate oxide is thicker at the ends of the gate for the purposes of reducing off-state leakage, increasing reliability and enhancing overall device performance. In one embodiment of the present invention, the MOS transistor is fabricated on a semiconductor substrate together with other devices in an integrated circuit (IC), using conventional IC fabrication steps. With some modification of one or more masks in the conventional IC processing steps, a nonuniform gate oxide is created that is thicker near the ends of the conducting channel of MOS transistor. Therefore, the present invention provides a MOS transistor with reduced leakage currents, and does not require additional complicated manufacturing steps for making the MOS transistor.